Polymer compositions based on polystyrene resins or polyphenylene ether (PPE) resins are commercially attractive materials. Many polystyrenes, for example, are characterized by high strength, impact resistance, and insulating capacity--both thermal and electrical. They are used for appliances, food containers, packaging, and for many other products.
PPE resins also have a wide variety of desirable attributes. They possess good chemical and physical properties, such as impact strength, hydrolytic stability, and high temperature-resistance. Moreover, they can be easily formed into films, fibers, and molded articles. One of the most popular compositions of this type is based on a blend of PPE with various types of styrene resins, such as rubber-modified, "high impact styrene".
Despite their many attributes, PPE compositions and polystyrene compositions are sometimes unsuitable for exterior applications, such as an outdoor environment. The "weatherability" of articles based on these materials is not always at an acceptable level, since the materials tend to exhibit yellowing when exposed to ultraviolet light. In addition to the discoloration, the yellowing can in turn act as the source of cracks which can weaken or permanently damage the article. PPE articles are therefore sometimes painted to shield them from UV sources.
Other techniques could be used to provide weatherability for PPE or polystyrene articles. For example, a thin layer of a weatherable material such as poly(vinylidene fluoride) (PVDF) could be applied to the surface of the article. In this manner, the article would continue to possess the desirable properties of the base material (i.e., the PPE or polystyrene), while also exhibiting the characteristics often required for an outdoor environment. As an example, U.S. Pat. No. 4,563,393 (Kitigawa et al) describes the use of a layer of a vinylidene fluoride resin to protect a layer of a thermoplastic resin such as PPE or polycarbonate from the forces of an outdoor environment. Similarly, it may sometimes be desirable to apply a layer of a PVDF-type material to a base layer of polystyrene, for weatherability or some other purpose.
It is usually critical that the protective layer adhere very tightly to the base layer. In the case of layers formed from chemically dissimilar polymers like PVDF with either PPE or polystyrene, satisfactory adhesion between the layers is not easily obtained. The Kitigawa et al invention relies on the use of polymerized units of an ethylenic unsaturated carboxylic acid ester to improve adhesion between the vinylidene resin and the thermoplastic base layer. The carboxylic acid ester units can be in the form of a copolymer-constituent of the base layer polymer, as an example.
There continues to be a need for additional techniques for improving the adhesion between a base layer of a PPE-based or polystyrene-based material, and an overlying layer (e.g., a protective layer) made from a material which is normally incompatible with PPE and polystyrene, such as a fluoride-based polymer. The adhesion should be strong enough to prevent any substantial peeling of the protective layer during rigorous conditions of use, e.g., in an outdoor environment exposed to a considerable amount of UV light. Moreover, the technique should be cost-effective and relatively simple to utilize in a larger-scale, industrial fabrication setting, such as a commercial molding facility. Furthermore, the adhesion technique should not adversely affect any of the desired properties for the completed article, e.g., the impact strength of a PPE/styrene base layer composition, or the weatherability of the protective layer.